Water-absorbent resins are widely used for hygienic materials such as disposable diapers and sanitary napkins, daily commodities such as pet sheets, and industrial materials such as water blocking materials for cables.
The hygienic materials such as disposable diapers and sanitary napkins generally comprise a top sheet, a back sheet, a hot melt adhesive, an elastic material, a water-absorbent resin, pulp fiber, and the like, and a number of synthetic resins and reforming agents are used therein. Thus the hygienic materials have some odor originating from the raw material components. Since these hygienic materials are worn on a human body, the odor makes those who wear them uncomfortable even if it is subtle, and therefore it is desired to develop an odor-free material.
Among the constituents of the hygienic materials, the water-absorbent resin has a subtle odor originating from the substances used in the production process, and since the odor tends to diffuse upon absorbing water, it is considered to be desirable to reduce the odor.
Known water-absorbent resins used for the hygienic materials include, for example, a partially-neutralized product of polyacrylic acid, a neutralized product of a starch-acrylic acid graft polymer, a hydrolysate of a starch-acrylonitrile graft copolymer, a saponified product of a vinyl acetate-acrylic acid ester copolymer, and the like.
As the process for production of such the water-absorbent resin, an aqueous polymerization method, a reversed-phase suspension polymerization method, and the like are known, but in the case of the water-absorbent resin prepared by the reversed-phase suspension polymerization method, in which polymerization is performed by suspending a water-soluble monomer in a dispersion medium, a major cause of the odor is considered to originate from the dispersion medium.
Known conventional arts for producing the water-absorbent resin by the reversed-phase suspension polymerization method include a method of polymerizing an α,β-unsaturated carboxylic acid and the aqueous alkali metal salt solution thereof in a petroleum hydrocarbon solvent using a radical polymerization initiator in the presence/non-presence of a crosslinking agent, in which a sucrose fatty acid ester is used as a protective colloid agent (see Patent Document 1), and a method of polymerizing an α,β-unsaturated carboxylic acid and a 25% by weight or higher aqueous alkali metal salt solution thereof in a petroleum hydrocarbon solvent using a radical polymerization initiator in the presence/non-presence of a crosslinking agent, in which a polyglycerol fatty acid ester with an HLB of 2-16 is used as a surfactant (see Patent Document 2), but these production technologies did not focus on reduction of odor, and thus the resultant water-absorbent resins were not those with low enough odor.
Patent Document 1:
Japanese Unexamined Patent Publication (Kokai) No. 61-87702
Patent Document 2:
Japanese Unexamined Patent Publication (Kokai) No. 62-172006